My copending application Serial No. 114,565 disclosed a conductive adhesive for biomedical electrode applications made by an improved solventless process. The adhesive is synthetic, dermally-nonirritating, conformable, cohesive, ionic and hydrophillic. The process by which the electrode is made involves the steps of: (1) forming an adhesive precursor comprising (a) a water-soluble polyhydric alcohol which is liquid at room temperature, (b) an ionic unsaturated free-radically polymerizable material which is soluble in the polydric alcohol, (c) a free radical initiator which is soluble in the polyhydric alcohol, and (d) a multi-functional unsaturated free radically polymerizable cross-linking agent; (2) coating the adhesive precursor on one side of an electrode plate (conductive sensing element); and (3) polymerizing the coated precursor in situ.
In the preferred embodiment of the conductive adhesive of the aforementioned disclosure, the ionic monomer is acrylic acid neutralized with an inorganic base such as potassium hydroxide.
The conductive adhesives of my previous disclosure are especially useful in electrosurgical grounding plate electrodes. They offer significant advantages over prior art conductive adhesives such as those described by Berg in U.S. Pat. No. 4,066,078. Berg discloses two classes of conductive adhesives plasticized with a polyhydric alcohol, viz., (1) polymers or copolymers derived from the polymerization of an ester of an olefinically unsaturated carboxylic ester and an alcohol having a quarternary ammonium group, and (2) sulfated cellulose esters. The processes by which these adhesives are formed into electrodes are much more tedious and expensive than those described in my previous disclosure and do not result in as good overall adhesive properties. The conductive adhesives of my previous disclosure are also an improvement over those specifically described in the copending application of Larimore et al, Ser. No. 155,191, filed June 2, 1980, now U.S. Pat. No. 4,352,359. The Larimore conductive adhesives may be formed from similiar ionic monomers, but a more expensive process is used and no crosslinked polymers are disclosed. Crosslinking allows for higher amounts of polyhydric alcohol without reducing viscosity below acceptable levels. A higher polyhydric alcohol level enhances hydrophilicity, thereby improving electrical conductivity.
Although the conductive adhesives of my previous disclosure provide significant improvements over the prior art, particularly when used in grounding plate electrodes, one problem has been encountered with their use in ECG electrodes. Electrodes utilizing my prior conductive adhesives do not recover satisfactorily following a defibrillation overload when used in disposable ECG electroces. Their polarization potential is too high to meet standards proposed by the Association for the Advancement of Medical Instrumentation (AAMI).
I have now discovered that by the addition of ionic salts, preferably those containing a halide ion, to the electrically-conductive adhesives of my prior disclosure, I am able to produce non-polarizing electrodes. Since the added salts provide the conductivity needed for good electrical performance, inclusion of an ionic monomer in the adhesive precursor is no longer necessary.